Some-remarks-on-direct-solid-fuel-combustion-using-chemical-looping-processes_2012_Current-Opinion-in-Chemical-Engineering.pdf

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Some remarks on direct solid fuel combustion using chemicallooping processesLiang Zeng, Mandar V Kathe, Elena Y Chung and Liang-Shih Fan

Direct solid fuel combustion using the concept of chemical

looping is a novel approach projected to have higher energy

efficiency than conventional and competing technologies in a

carbon-constrained scenario. The concept is first discussed by

presenting a thermodynamic analysis on oxygen carrier

material selection. Iron-based material is considered as a

promising candidate, and its salient features in the context of

solid fueled chemical looping combustion (CLC) are

introduced. The CLC reactor configurations are compared

based on the modes of reducer design and operation, which

significantly affects the whole system performance. A brief

review on recent experimental studies indicates that initial

results for the solid fueled CLC technologies are promising. An

optimal reducer configuration that provides effective gas–solid

contacting pattern, and the need for favorable thermodynamics

of the oxygen carrier medium were identified as the important

challenges for an eventual chemical looping combustion with

solid fuels commercial scale-demonstration.

Address

William G. Lowrie Department of Chemical and Biomolecular

Engineering, 140 W 19th Avenue, 125 A Koffolt Labs, The Ohio State

University, Columbus, OH 43210, United States

Corresponding author: Fan, Liang-Shih ([email protected],

[email protected])

Current Opinion in Chemical Engineering 2012, 1:290–295

This review comes from a themed issue on Reaction engineering

and catalysis

Edited by Theodore T Tsotsis

For a complete overview see the Issue and the Editorial

Available online 20th June 2012

2211-3398/$ – see front matter, # 2012 Elsevier Ltd. All rights

reserved.

http://dx.doi.org/10.1016/j.coche.2012.05.001

IntroductionSolid carbonaceous fuels, including coal and biomass, are

projected to continue to be a major energy source in the

future [1]. However, the CO2 emissions from the existing

technologies extracting energy from coal are leading to

adverse effects on the atmosphere [2]. The utilization of

biomass is an approach to mitigate CO2 emissions, but the

employment of novel energy technologies is highly

desired to enhance the low energy density biomass con-

version [3]. Amongst the new technologies, chemical

looping is projected to offer the maximum cost-reduction

benefit in a carbon-constrained scenario [4,5]. It has the

potential to achieve above 90% CO2 capture with a cost of


electricity (COE) increase of about 30% when applied to a

pulverized coal (PC) firing power plant [6��]. In compari-

son, the COE increase for a conventional PC plant with an

amine scrubbing system is more than 80%. [5]. This novel

technology can be applied for electricity production from

chemical looping combustion (CLC) [7–10,11��], or for

hydrogen, fuel and chemical synthesis from chemical

looping gasification (CLG) [9,10,11��,12–15].

A typical CLC scheme consists of a reducer and a com-

bustor, with an oxygen carrier (OC) stream circulating in

between. In the reducer, the fuel reacts with the OC,

typically a metal oxide, oxidizing the fuel to form a

mixture of CO2 and H2O, while reducing the metal oxide

as shown in Reaction (1). The reduced metal oxide is sent

to the combustor where it is reacted with air to re-oxidize

the metal oxide for the next looping cycle as shown in

Reaction (2). The oxidation reaction of metal oxide with

air is exothermic, and the spent-air is used to generate

electricity. The OC particles act as a medium for both

oxygen and heat for the reactor system.

MeOxþ CðfuelÞ ! CO2þ MeOyðy < xÞ (1)

MeOyþ O2ðairÞ ! MeOx (2)

There is a considerable amount of work reported on CLC

technologies using gaseous fuels such as synthesis gas

derived from coal gasification and natural gas [9,13–19].

However, direct solid fuel conversion in the reducer has

better efficiency and economics compared to using its

gaseous derivatives, which is mainly attributed to elim-

inating the energy/cost penalty associated with the sep-

arate solid fuel gasification step. This work discusses the

fundamental aspects involved in the direct solid fuel CLC

process including a brief thermodynamic analysis of

selecting an OC, and a summary of iron-based material

development, reactor configurations and integrated sys-

tem demonstrations.

Oxygen carrierPrimary material selection

The reduction–oxidation (redox) behavior of the OC

largely depend on the thermochemical properties of its

principal metal oxides and can significantly affect the

system performance. Thus, the primary OC material

selection focuses on the metal oxides that have favorable

equilibria with both solid-fuel and air, which can be

conducted with the assistance of a consolidated Elling-

ham diagram, as shown in Figure 1 [19,20]. The Elling-

ham diagram illustrates the dependency of the Gibbs free

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Remarks on solid fuel chemical looping Zeng et al. 291

Figure 1

0

-20

-40

-60

ΔG, k

cal/m

ol O

2

-80

-100500 600 700 800 900 1000

T, ºC

CO2

COFeTiO3

FeO

Fe3O4

CaSO4

NiO

PO2 = 0.21 atm, Ambient Air

P CO/P CO2

= 5:995

PO2 = 0.01 atmMn2O3

CuO

Cu2O

Fe2O3

Mn3O4

H2O

1100 1200

Car

bon

Con

vers

ionMod

e 2

Ran

geMod

e 1

Ran

ge

Ful

l Fue

lC

onve

rsio

n

Ful

l Air

Con

vers

ion

CLO

U

Hyd

roge

nP

rodu

ctio

n

1300

P CO/P CO2

= 1:999

Current Opinion in Chemical Engineering

Ellingham diagram calculated from HSC Chemistry, showing the

variation of oxidation Gibbs free energy with temperature for various

compounds [19,20].

energy (DG) of the redox reactions on temperature, where

evaluating the position of lines can indicate the relative

reduction and oxidation potentials of OC materials.

For the reducer operation, in order to effectively convert

solid carbonaceous fuels, the CO2 line should be posi-

tioned below the primary material lines in Figure 1. Based

on this criterion, metal oxides such as CuO, NiO and

Fe2O3, and metal sulfates such as CaSO4 are normally

selected for various CLC applications [21,22,23�]. At the

same time, the candidate lines must be higher than the

PCO/PCO2 = 1:999 line, which represents a 99.9% fuel

conversion, in order to obtain pure CO2 and H2O at

the reducer gas outlet. Figure 1 reveals that NiO and

CaSO4 cannot achieve >99.5% fuel conversions. Further-

more, cost-inefficiency and high-toxicity of NiO materials

are generally considered disadvantages for large-scale

industrial applications. Although CaSO4 has a high theor-

etical oxygen carrying capacity (OCC) and low cost, it is

negated by CaO formation and sulfur emissions during

the redox process [24].

For the combustor operation, the material lines need to be

below the PO2 = 0.01 atm line in order for full air conver-

sion. The materials near this line can also release free

oxygen to boost solid fuel conversion in the reducer,

which is called Chemical Looping Oxygen Uncoupling

(CLOU) process [25–30]. Figure 1 indicates that the

CLOU materials have very narrow operation range for

the oxidation step, which usually requires long residence

time and high excess air flow. For example, Mn2O3 line is

above the line that corresponds to an oxygen partial


pressure of 0.21 atm. Thus, it is difficult to regenerate

Mn2O3 using ambient air. As a result, the Mn3O4–MnO

cycle is usually not considered for chemical looping

applications as it is limited by its 7% theoretical OCC.

The low melting point of Cu, low strength, high percen-

tage of support materials, high amount of excess air and

long regeneration times may pose serious challenges for

CuO material development [30].

From Figure 1, iron oxides exhibit favorable thermodyn-

amic properties with complete conversions for both fuel

and air in the redox cycles. Furthermore, iron oxide is

advantageous for CLC as iron-based OCs possess low raw

material cost, high melting points, high mechanical

strength and less environmental and health concerns.

The following sections further discuss iron-based

materials and process developments.

Iron-based materials

Based on the Ellingham diagram, Fe2O3 can convert

100% carbonaceous fuel to CO2 and H2O. However,

Fe3O4 and FeO lines are close to H2O and CO2 lines;

thus, incomplete fuel conversion occurs when Fe3O4 or

FeO are at the last stage of contact with the fuel. Several

counter examples have been reported [31], which are not

in compliance with the thermodynamic constraints.

Because of iron’s multiple oxidation states, the gas–solid

contacting patterns in the reducer dominate both the fuel

and the OC conversion. In a mixed flow pattern, or Mode

1, in order to achieve 100% CO2 at the reducer gas outlet,

only the Fe2O3–Fe3O4 redox pair can be used. In com-

parison, a countercurrent flow pattern, or Mode 2, could

create multistage equilibrium states across the reducer. At

the top of the reducer, Fe2O3 fully oxidizes fuel into CO2

and H2O; while at the bottom, the solid fuel is gasified,

and the OC is reduced to FeO and Fe. Both experimental

and simulation results show that the maximum iron oxide

conversions, while ensuring 100% fuel conversion, are

11.1% and >50% for a well-mixed mode and for a coun-

tercurrent mode, respectively [9,32]. The Mode 2 design

thus results in a minimal OC circulation rate in the iron-

based CLC system. Figure 1 also illustrates that FeO,

Fe3O4 and H2O lines are close to each other, which

indicates the possibility for hydrogen production in a

CLG process [9].

Naturally occurring ores such as ilmenite (FeTiO3) have

been considered for CLC application in light of its low

cost and one step reduction from Fe2TiO5 to

FeTiO3[23�,33,34]. However, phase separation and seg-

regation occur during the redox cycles, which results in

separate iron oxide and titania phases [35]. The iron phase

migrates to the surface during the oxidation of FeTiO3,

while the TiO2 crystals increase over the redox cycles,

which could fracture the ilmenite particle. This mechan-

ism could lower the particle strength, and the counter

mechanism has been applied to TiO2 production in the


292 Reaction engineering and catalysis

Figure 2

combustor gas

fuel reaction products

air

Risercombustor

combustor gas

Reducer

fuel/reducing gas

Riser combustor

airfuel/reducing gas

fuel reaction products

combustor gascombustor gas

OC

OC

OC

OCOC

OC

Reducer

Current Opinion in Chemical Engineering

(a) Mode 1 reducer: fluidized bed gas–solid flows; (b) Mode 2 reducer: gas–solid dense phase/moving bed flows.

Becher process [36]. Figure 1 also shows that FeTiO3 line

is far away from H2O line, meaning that steam oxidation

of FeTiO3 for hydrogen production is not favored.

Pure iron oxide quickly loses its reactivity in the first

several redox cycles. When inert support materials are

added, the strength, reactivity and recyclability of OC

particles could be drastically improved [9,23�,37]. The

lack of understanding of the complex solid–solid inter-

action between the primary metal and the support

material magnifies the inability to predict the influence

of support/active metal oxide combinations on OC per-

formance; thus, much of OC studies are by trial-and-error.

A recent study on ionic transfer mechanism opens a new

direction for OC design and synthesis [38,39].

Iron-based CLC reactor designA chemical looping reactor system consists of a reducer, a

combustor, a non-mechanical valve, a fine removal

device, gas–solid separators, and other auxiliaries, where

Table 1

Reducer design comparison Mode 1 versus Mode 2

Reducer modes

Operation regime Bubbling

or spout

Gas solid contacting pattern Mixed

Controllability on fuel and OC conversions Poor, du

gas cha

Maximum iron oxide conversion 11.1% (t

Solids circulation rate High

Subsequent hydrogen production No

Particle size, mm 100–600

Reducer gas velocitya, m/s <0.4

Reactor size for the same fuel processing capacity Large

Hydrodynamics effects on scaling up Large

a Reducer gas velocity calculated at 900 8C, 1 atm.


the reducer is the key component for fuel conversion. As

discussed in the previous section, there are two funda-

mentally different reducer modes that classify the CLC

systems. Figure 2 illustrates that Mode 1 reducer design is

generally characterized by mixed fluidized beds, while

Mode 2 is characterized by gas–solid countercurrent

moving/dense beds. Table 1 further compares the two

modes in the context of iron-based CLC applications.

The channeling flow and back mixing in the Mode 1

reducer usually result in poor fuel and OC conversions.

The Mode 2 reducer design has the distinct advantage of

enhancing the iron oxide conversion while enabling full

fuel conversion. Thus Mode 2 reactor requires a signifi-

cantly lower circulation rate of Fe2O3 particles than Mode

1. The small circulation rate together with the mitigating

circumstances in the moving bed reactor reduces the

particle attrition rate and OC loss. Furthermore, in the

Mode 1 system, it is inevitable that the sizes of the OC

particles, attrited fines, the solid fuel and ash, are similar,

which complicates the ash removal and reactor design.

1 2

, turbulent, fast fluidized,

ed bed

Moving packed, or multistage

fluidized bed

Countercurrent

e to back mixing and

nneling

High

o Fe3O4) >50% (to Fe and FeO)

Low

Yes

[40,41] 1000–3000 [9,37]

>1.0

Small

Small



Table 2

Solid fueled chemical looping combustion process studies

Organization Type of fuel, solid-(fuel processing

input capacity -kWth)

System characteristics

(reducer configuration,

type, oxygen carrier)

References

Southeast University, Nanjing, China Biomass pine-sawdust (10-kWth) Interconnected spout fluidized bed-BFB,

Mode 1, Fe2O3-Fe3O4 based loop

[40,41]

Chalmers University of Technology, Sweden Coal, pet-coke, char (5–10 kWth) Circulating fluidized bed-BFB, Mode 1,

Ilmenite loop

[34,42–45]

Instituto de Carboquı́mica (ICB-CSIC), Spain Coal (5-kWth) Bubbling fluidized bed-BFB, Mode 1,

Ilmenite loop

[33,46,47]

The Ohio State University, USA Met coke, PRB, bituminous coal

(25-kWth)

Dense moving bed – entrained fluidized

bed, Mode 2, Fe2O3–FeO/Fe loop

[6��,9,48]

Additional compartments, such as bubbling beds, are

usually used for ash and unreacted fuel segregation and

separation in the Mode 1 system. A Mode 2 moving bed

reactor allows for a larger particle size of OCs, which eases

the ash separation and requires a much smaller reactor

diameter as higher gas velocity in the reactor is possible.

Table 2 summarizes the major large scale iron-based solid

fuel CLC process studies conducted worldwide.

Mode 1 based iron-looping systems

Shen et al. [40] constructed and operated a 10-kWth

biomass based CLC unit. The reducer is in a Mode 1

configuration featuring a spout-fluidized bed mode, while

the combustor is a fast-fluidized bed. The OC used is an

iron-based powder (300–600 mm) where its oxidation

state is cycled between Fe2O3 and Fe3O4. The results

show that the dominant reaction in the reducer is between

the gasified biomass and the OC, with the gasification of

biomass as the rate limiting step. At steady operation, an

overall carbon-capture efficiency of 44–54% was reported.

The low carbon-capture efficiency was attributed to

inadequate biomass residence time in the reducer and

the intermixing of biomass from the reducer with the

combustor owing to an inefficient inner-seal. Improving

on Shen’s biomass CLC concept, Gu et al. [41] incorp-

orated an external loop-seal to the spout-fluidized bed

reducer. They used 100–300 mm naturally occurring

hematite ore as the OC medium. A 1-kWth biomass feed

CLC unit was constructed giving a carbon-capture effi-

ciency between 60 and 80%. They also addressed the

issue of handling biomass-ash by blending in coal in a co-

injection scenario. A 1:1 co-injection scenario for a 1-kWth

energy input gave a promising carbon-capture efficiency

of 85–95%. In addition, Gu et al. found that naturally

occurring iron ore is thermodynamically constrained caus-

ing loss in conversion efficiency with increasing tempera-

tures.

Berguerand and Lyngfelt [34,42–45] extensively studied

various solid fuels (e.g. petcoke, coal and biomass) in

a 10 kWth circulating fluidized bed CLC unit using

ilmenite. The OCs react with the volatile matter from


the solid fuel in a low-velocity section. A carbon stripper is

used to recover the un-reacted coal from the particles

while allowing the OC to overflow to the combustor. A

third high-velocity section entrains a portion of the OC

and unconverted coal to a cyclone to be recycled to the

reducer increasing the residence time of the solids. The

average fuel conversion of the system was estimated to be

70–80% with a CO2 capture efficiency of 60–70%. The

system gave continuous stable hydrodynamic flows for

the ilmenite, indicating the natural mineral’s potential as

an economical and durable OC candidate. Cuadrat et al.[33,46,47] have designed a bubbling fluidized bed system

for processing 5-kWth bituminous coal input with ilme-

nite as the OC. The results show that the carbon-capture

efficiency increases as the OC flow-rate is reduced to be

equivalent to solid-fuel flow rate, re-emphasizing the

point that a higher solid-fuel residence time is essential.

They have shown that co-injection of limestone with

ilmenite helps the reaction kinetics where the removal

of the gaseous products promotes an increased fuel-con-

version [47].

Mode 2 based iron-looping systems

Fan et al. has developed the coal direct chemical looping

(CDCL) process based on a Mode 2 reactor design, and

demonstrated the moving bed concept on both a 2.5-kWth

bench scale and a 25-kWth subpilot scale [6��,9]. In this

configuration, fresh Fe2O3-composite particles are fed

from the top of the moving bed reducer, while CO2 is

used to pneumatically convey pulverized coal to the

middle section of the reactor. A small amount of CO2

and/or steam is also introduced at the bottom of the

reducer to enhance char conversion. The coal injection

port divides the reducer into two sections. The upper

section (Stage I) ensures full conversion of gaseous

species to CO2 and H2O, whereas the lower section (Stage

II) is used to maximize char and iron oxide conversions.

The unique features are the presence of a moving-bed

reactor that is designed to account for the volatiles from

coal and the required residence time to gasify the char for

high carbon-capture efficiency. Extensive 2.5 kWth bench

scale testing has shown >99.9% volatile conversion and


294 Reaction engineering and catalysis

>95% char conversion in Stage I and II, respectively. The

25 kWth subpilot scale system has been successfully

operated for over 200 h with different kinds of coal fuels.

Through the operation, the concepts of integrated CDCL

process were verified, and the control systems were

examined and data were obtained for scaling-up to pilot

and commercial sized systems. The current design and

operating condition have yielded over 80% metallurgical

coke conversion with nearly 100% CO2 purity.

Concluding remarksAn optimal metal oxide particle is selected according to its

thermodynamic equilibrium, recyclability, synthesis

method, resistance to attrition, and most importantly

the materials cost. By comparing major candidates, iron

oxides are considered as feasible OC materials. The iron-

based OCs have the maximum potential for optimizing

the efficiencies in the solid fuel chemical looping system.

The design of iron-based CLC systems can be classified

on the basis of the fuel–OC contacting pattern in the

reducer. Mode 1 contacting pattern refers to a mixed flow

mode, typical of a fluidized bed reactor system. One

equilibrium stage can be achieved in the Mode 1 oper-

ation, which yields an iron oxide conversion of 11.1%.

Mode 2 refers to the fuel–OC countercurrent contacting

pattern. This is represented in cases where the reactor is

operated in a dense moving-bed or a multistage fluidized

bed simulating multiple equilibrium stages. Compared

with Mode 1, Mode 2 operation is possible to have higher

OC conversion that can reduce the solids circulation rate

and increase the system efficiency. The main consider-

ations in designing a reducer for a solid fuel CLC appli-

cation are sufficient fuel residence time and kinetics of

the solid-fuel gasification. Additional considerations are

necessary for contaminants (such as ash and sulfur)

removal from the reactor system.

References and recommended readingPapers of particular interest, published within the period of review,have been highlighted as:

� of special interest�� of outstanding interest

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6.��

Fan L-S, Zeng L, Wang W, Luo S: Chemical looping processes forCO2 capture and carbonaceous fuel conversion – prospect andopportunity. Energy Environ Sci 2012 doi: 10.1039/c2ee03198a.

This perspective paper generalizes the key aspects of chemical loopingtechnology including the desired particle characterization, recent pro-


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11.��

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Some-remarks-on-direct-solid-fuel-combustion-using-chemical-looping-processes_2012_Current-Opinion-in-Chemical-Engineering.pdf